1. Field of the Invention
This invention relates to a base station selecting method, a reference signal receiving method and subscriber apparatus, and more particularly, is applicable to a pilot signal detector, a pilot signal detecting method and a mobile communication terminal equipment which are used in a mobile communication system for carrying out a communication by, for example, a code division multiple access (CDMA) method.
2. Description of the Related Art
Conventionally, when one base station is shared with many subscribers to carry out a communication, there have been utilized technical methods such as a frequency division mode, a time-division multiplex mode and, a code division multiple access mode, in order to avoid the interference between the communication lines of the respective subscribers. These methods respectively have advantages and disadvantages and are respectively selected depending on the purpose of the communication system.
For example, according to the code division multiple access (referred to as a CDMA (Code Division Multiple Access), hereinafter) method, specific codes (referred to as a PN (Pseudo random noise sequence) code, hereinafter) assigned for respective lines are employed so that the modulated waves of the same carrier frequency are diffused to a band wider than an original frequency band (this is called a spectrum diffusion, hereinafter) and the respective modulated waves subjected to a spectrum diffusion processing are multiplexed and the obtained multiplexed modulated waves are transmitted. Further, the received spectrum diffusion signals are respectively synchronized with the PN code supplied through a line to be demodulated Thus, only a desired line is identified.
Specifically stated, a transmitter side initially assigns different PN codes to respective lines. In this case, the PN code is composed of a series of pseudo random number codes. The transmitter side respectively multiplies the modulated waves transmitted through the respective lines by respectively different PN codes and the obtained modulated waves are subjected to a spectrum diffusion processing. In this connection, the respective modulated waves are subjected to a prescribed modulation processing before they undergo the spectrum diffusion processing. In this way, the modulated waves respectively subjected to the spectrum diffusion processing are multiplexed and the obtained multiplexed modulated waves are transmitted.
Then, in a receiver side, the receiving signals transmitted from the transmitter side are respectively synchronized with and multiplied by a PN code the same as that assigned to the line to be demodulated. Thus, only the modulated wave transmitted through an object line is demodulated.
In this manner, according to the CDMA mode or method, only when the same PN code is commonly set to the transmitter side and the receiver side, a communication can be directly carried out there between for each call. Further, according to the CDMA mode or method, since the respectively different PN codes are employed for respective lines so that the modulated waves are subjected to a spectrum diffusion processing, the receiver side can demodulate only the spectrum diffusion signal transmitted through a line to be demodulated. Still further, since the PN code is composed of a series of pseudo random number codes, it may be said that the CDMA mode is excellent in secrecy.
Besides, according to a mobile communication system using the CDMA mode or method, the base stations of a transmitter side repeatedly transmit PN codes (called a pilot signal, hereinafter) in order to obtain and maintain the synchronization of a subscriber side and reproduce clocks. The subscriber of a receiver side detects the respective pilot signals transmitted from a plurality of base stations and assigns the detected timings respectively to demodulators. In this case, the subscriber generates the PN codes in the respective demodulators. The demodulators respectively multiply the PN codes by a spectrum diffusion signal transmitted from an object base station at timings respectively assigned to the demodulators to thereby demodulate appropriately the spectrum diffusion signal.
Specifically described, according to the mobile communication system utilizing the CDMA mode or method, the respective base stations transmit the PN codes with respectively different timings as the pilot signals. The subscriber side detects the timing of the pilot signal supplied from the object base station, and synchronizes the PN code generated in the demodulator with the timing, so that the subscriber can properly demodulate only the spectrum diffusion signal transmitted from the target base station.
Parenthetically, as described above, the respective base stations transmit the PN codes at respectively different timings, however, the PN codes themselves are respectively formed in the same code pattern. In other words, the differences in timing between the different pilot signals for the respective base stations directly correspond to the differences between the PN codes.
In the meantime, the above mentioned subscriber carries out a mobile communication, so that it moves while it switches base stations as objects to communicate with the subscriber. Such a switching of the base stations is called a hand-off. In order to achieve an efficient hand-off, the subscriber is designed to continuously monitor the timings of the pilot signals of base stations (referred to as adjacent base stations, hereinafter) except a base station which is an object to currently communicate with the subscriber.
According to the CDMA mode or method, the adjacent base stations are monitored by detecting the timings of the pilot signals, that is, phases. Thus, the base stations to be monitored are divided into groups based on a prescribed priority for monitoring to apply a proper system to these base stations.
However, it has been inconvenient that a specific standard for selecting base stations to be monitored is not stipulated in the CDMA mode or method. Therefore, it has been necessary to obtain a simple and efficient method and apparatus for selecting a base station to be monitored in order to lower the power consumption of a subscriber.